In the pneumatic radial tire running under loading, a phenomenon that a part of a sidewall portion located just above a tread contacting with a road surface is largely bent and also a part of a bead portion located outward from a rim flange of a wheel in a radial direction of the tire is largely deformed outward in a widthwise direction of the tire as shown in FIG. 21 repeatedly occurs every ground contact. By such a deformation phenomenon is applied a large shearing strain to a neighborhood of an outer end of a turnup portion of a radial carcass between the carcass and rubber located therearound in the widthwise direction of the tire and hence there is a fear of deteriorating the durability of the tire.
And also, a deformation in substantially a circumferential direction of the tire is created in the sidewall portion and the bead portion corresponding to a leading part and a trailing part in a ground contact face of the tread portion, so that the shearing strain is also applied to a position of the outer end of the turnup portion of the carcass and hence there is a fear of deteriorating the durability of the tire.
In the heavy duty pneumatic radial tire applied to a truck, a bus and the like, a recommended internal pressure applied after the assembling into a rim is set to a higher level. However, in case of applying such a higher internal pressure, compression force is applied to rubber part of the bead portion sandwiched between the turnup portion of the carcass ply in the bead portion and a flange of the rim (for example, a rubber reinforcing layer contacting with the rim) by reaction force of the internal pressure, whereby the rubber part is moved outward along the rim flange in the radial direction of the tire in proportion to the deformation quantity corresponding to the compression force. Since the carcass ply reinforced by the high rigidity cords such as steel cords or the like hardly deforms, a large shearing strain is created in an outer surface rubber constituting the carcass ply located on the turnup portion thereof, and as a result, the shearing strain applied to the outer end of the turnup portion of the carcass ply becomes further larger.
As means for controlling the shearing strain created at the outer end of the turnup portion of the carcass ply, it is useful that at least one reinforcing layer is arranged so as to locate an outer end thereof outward from the outer end of the turnup portion in the radial direction of the tire to thereby transfer trouble nucleus from the outer end position of the turnup-portion to the outer end position of the reinforcing layer, which is disclosed, for example, in JP-A-11-20423 and the like.
As another means for controlling the shearing strain, it is useful to rationalize a bead filler as disclosed in JP-A-8-225005.
Even in recent heavy duty pneumatic radial tires, however, the flattening of a section profile is promoting, and an input to the bead portion in the running tends to be increased by such a flattening. In addition, the durability of the bead portion can not be sufficiently improved by the aforementioned means for controlling the shearing strain in the retreading operation (the retreading number also increases), and hence further improvement is desired.
In the construction shown in JP-A-11-20423, troubles may be still caused in the outer end position of the turnup portion of the carcass and there is a problem that the protecting action of the reinforcing layer to portions of the main trouble is not sufficient.
In low-section profile, large-size tires having an aspect ratio of not more than 60%, there is a serious problem that premature deterioration of new tire is caused by the trouble of the bead portion but also the retreading number decreases.